A typical goal of charged particle systems is to focus a particle beam into a small spot. This goal is accomplished using a column of magnetic and electrostatic lenses that set the focal properties of the beam. A cross-section of an exemplary column of magnetic and electrostatic lenses is shown in FIG. 7. Such a column includes an objective lens 706, a double deflection coil 702, a stigmator 704, and a beam limiting aperture 708. The column is disposed along a longitudinal axis a-a; the beam travels along this axis.
There are many imperfections in tools that use charged particle beams. These imperfections cause, in turn, imperfections in the focusing properties of the tool. One imperfection that prevents sharp focus is referred to as astigmatism. The causes of astigmatism may be traced to many sources: lens aberrations, mechanical misalignments, and particle contaminants, to name just a few. Specifically, astigmatism results in the focal length in some direction transverse to the direction of the charged beam to be different from the focal length in the orthogonal transverse direction. Simultaneous best focus in the two transverse directions (best focus) is thereby prevented. An example of the effects of astigmatism is shown in FIG. 8. FIG. 8(a) shows a gray scale image of a sample contact hole in optimal focus; FIGS. 8(b), 8(c), 8(d), and 8(e) show stigmated images of the same contact hole. FIG. 8(b) shows blurring of the contact hole image in the -45.degree. direction transverse to the direction of the charged beam. FIG. 8(c) shows blurring of the contact hole image in the +45.degree. direction transverse to the direction of the charged beam. FIG. 8(d) shows blurring of the contact hole image in the vertical direction transverse to the direction of the charged beam. FIG. 8(e) shows blurring of the contact hole image in the horizontal direction transverse to the direction of the charged beam.
To combat the problem of astigmatism, many charged particle beam systems are fitted with adjustable stigmator coils. These compensation coils are used to correct the accumulated effects of astigmatism along the path of the beam. For unknown reasons, however, the correct compensator settings tend to change a periodically.
In particular, modern scanning electron microscopes (SEMs) are equipped with two sets of quadrupole compensation coils. FIG. 9 shows a schematic cross-section of a SEM 900. The SEM 900 includes: an electron gun 902, condenser lenses 904, scan coils 906, an objective lens 907, a cathode ray tube (CRT) 908, a scan generator 910, and an amplifier 912.
FIG. 10(a) shows a cross-section view of an eight-pole iron yoke that may be configured to produce two compensators. FIGS. 10(b) and 10(c) illustrate wiring schematics for the eight-pole iron yoke shown in FIG. 10(a). In many applications, such as semiconductor manufacturing, limitations in focusing properties or astigmatism in metrology tools is a major stumbling block. For example, as semiconductor manufacturing advances into the sub 0.18 .mu.m domain, the requirements for focusing and astigmatism correction of critical dimension-SEMs (CD-SEMs) metrology tools have become so severe that human operators can no longer make these adjustments with sufficient accuracy and repeatability. Therefore, the development of sound and consistent methodologies for determining proper compensator coil settings is crucial.
The difficulty with using quadrupole lenses, which is a consequence of Maxwell's laws, is that adjustment of the focusing properties in one direction causes defocusing in an orthogonal direction. In order to properly focus the beam, the settings of the quadrupole compensation coils should be adjusted in at least two directions simultaneously with an optimization of the main objective lens excitation.
Adjustments to corrector quadrupole lenses are often performed by human operators. Human operators use subjective criteria to determine the optimal quadrupole compensation settings. This presents serious difficulties: two different measurements performed by the same tool on the same specimen, each time having been adjusted by a different operator, yield two different results. Discrepancies between such measurement results may be intolerable with respect to an error budget. Error budgets are constantly narrowing, as pointed out above, to meet the new demands of semiconductor circuit technologies. Therefore, it is desirable to determine correct astigmation corrector settings by automated means, and by using objective, well-defined focusing or sharpness criteria.
Many charged particle beam systems are already equipped with an automated focusing routine for the objective lens. A block diagram outlining the steps involved in such an automated focusing routine is displayed in FIG. 11. In step 1102 a focus sweep is initiated. In step 1104 the focus setting of an objective lens is determined. In step 1106 a target is scanned and an image containing information about the target is obtained. In step 1108 the information contained in the image obtained is analyzed to determine certain sharpness measures. In step 1109 a test is performed to determine whether sufficient sharpness measure information has been obtained in order to detect a sharpness maximum. If so, the objective lens is set for maximum sharpness in step 1110. If the information obtained thus far in the focus sweep is insufficient, then the focus setting of the objective lens must be varied and the process of steps 1104-1110 repeated.
A block diagram illustrating the steps involved in a conventional astigmatism correction method is displayed in FIG. 12. In step 1202 the astigmatism algorithm is initiated. In step 1204 the setting of the X-astigmatism corrector is determined. In step 1206 a focus sweep such as, for example, the focus sweep of FIG. 11 is initiated. In step 1208 a test is performed to determine whether sufficient sharpness measure information has been obtained in order to set the X-astigmatism corrector properly. If the information obtained thus far is insufficient, then steps 1204, 1206, and 1208 are repeated. The function of steps 1210, 1212, and 1214 performed for the Y-astigmatism corrector are analogous to the function of steps 1204, 1206, and 1208 performed for the X-astigmatism corrector. Conventional astigmatism correction methods systematically step through a range of magnet currents for one of the astigmation correctors. At each setting of the corrector magnet current, a best focus is determined for the tool, e.g. a SEM, using the automated focus routine of the tool. The automated focus routine may work according to a number of different principles each corresponding to a sharpness measure such as, for example, contrast maximizing, maximizing high spatial frequency signal content, or the like.
Central to its application, however, is the systematic stepping through the objective lens current (OLC) as well as other steps taken to ensure that a sharpness measure curve with a clear maximum is obtained. At each setting, the equivalent of an image of a sample is obtained from the tool for analysis. Best focus corresponds to a maximum in the sharpness measure curve. Best astigmation corrector (magnet current) setting is then determined by finding the maximum best focus sharpness measure as a function of corrector magnet current. The process can then be repeated for the second astigmation corrector. For this process to succeed, it is important for the sample to contain edges along the principal axes of the two astigmation correctors. It is also important that the focus routine be sensitive to the sharpness of those edges.
Conventional astigmation correction methods may involve many applications of the automated focus routine of the tool (see FIG. 12); this repetition detracts from the efficiency of the method and may cause sample damage. In addition, astigmation correction by human operators may no longer be sufficiently accurate for current and future industrial applications. Proprietary automated astigmation correction routines may not, by their nature, provide the user of the tool with the precise criteria and methodology used in the computation of the astigmatism error and its correction. The effectiveness and appropriateness of use of such proprietary routines is hence difficult to evaluate.
These deficiencies of conventional and proprietary methodologies, and of human operators, invoke a need to determine efficiently proper astigmation corrector settings by automated means using objective, well-defined focusing or sharpness criteria. An object of the present invention is to provide a methodology for computing and correcting astigmatism by automated means in a charged particle beam system based on sharpness criteria. Another object of the present invention is to provide users of charged particle beam systems with objective means for comparison of astigmatism error measurement and correction made by, for example, proprietary routines or the like.